1. Applications of solid acid catalysts
Acid catalysts are widely used in many chemical processes, such as dehydration of alcohol, isomerization of olefin, isomerization of paraffin, alkylation, cracking, and polymerization etc. Different processes require different acidities of the acid catalysts. For example, in the reaction of producing an olefin by dehydration of an alcohol, a double bound isomerization product will be obtained if a weak acid catalyst is used in the reaction, and it takes a strong acid catalyst to obtain an olefin product with skeletal rearrangement, or an oligomer product of the olefin if the olefin formed has a high reactivity. In addition to attacking the double bound of the olefin, a strong acid catalyst also initiates a hydrogen transfer reaction and thus causes a conjunct polymerization of the olefin. Moreover, a strong acid catalyst can cause a saturated hydrocarbon undergo skeletal isomerization and alkylation. Even the most stable paraffin such as methane and ethane can be catalyzed to form a higher molecular weight hydrocarbon with a super strong acid catalyst. Some of the reactions which can be catalyzed by an acid catalyst have to be carried out at a relatively low temperature due to thermodynamic factors. For a reaction undergoing at a relatively low temperature, an strong acid catalyst having an acidity which approximates to 100% sulfuric acid is required. Sulfuric acid, hydrofluoric acid and phosphoric acid are thus often used in these reactions. However, using a liquid type acid catalyst generally involves problems such as corrosion of reactors and pipelines, and environmental pollution, etc. Therefore, there is always a need in the chemical industry to develop solid acid catalysts having strong acidity.
The acidity of a solid acid catalyst is expressed by Hammet acidity function, Ho, which is about equivalent to the pKa constant of the indicator compound. Solid acid catalysts have been extensively used in oil refinery and petrochemical industries in recent years, such as in the alkylation, isomerization, cracking and polymerization reactions. In general, most of the reactions via a reaction mechanism involving an intermediate of carbonation can be catalyzed by a solid acid catalyst. Among the common solid acid catalysts, various zeolites constructed by a matrix of silica and alumina are considered having good catalytic activities to many manufacturing processes in the oil refinery and petrochemical industries. The acidity of zeolite can reach a Ho value of -8.2, which is equivalent to 90% sulfuric acid. However, this acidity is still not strong enough to catalyze some reactions which have to be carried out at lower temperatures.
2. Synthesis of methyl tertiary butyl ether (MTBE):
Ethers, such as Methyl Tertiary Butyl Ether (MTBE), Ethyl Tertiary Butyl Ether (ETBE) and Methyl Tertiary Amyl Ether (TAME), are introduced into gasoline as octane boosting additives, because these ethers have high octane number ({RON+MONumber}/2 higher than 105), high combustion value, low water solubility (lower than 4.8%), low volatility (RVP=8 psi), and they are fully miscible with gasoline and less toxic. These ethers not only can make up the octane number lost by the reduced lead content of the unleaded gasoline, but reduce the amount of CO, NO and hydrocarbon compounds generated by the incomplete combustion of the gasoline due to oxygen contained therein. Among these ethers, MTBE is the most popular one due to its low price.
In 1907 Belgium chemist, Reychler first synthesized a tertiary ether by mixing trimethyl ethene, methanol and sulfuric acid [Reychler, Buul. Soc. Chim. Belg., 21 (1907) 71]. Up to the present, MTBE is synthesized similarly by reacting methanol and isobutene in the presence of an acid catalyst, wherein the acid catalyst used mostly is a Bro/nsted type solid acid catalyst except that BF.sub.3 [H. G. Shnelder and N. J. Roselle, U.S. Pat. No. 2,197,023.] and Pt salts [U.S. Pat. No. 3,718,701.] are Lewis acid. In the literature, the solid acid catalysts which have been used for synthesizing MTBE includes: Zeolite {H-ZSM, ZSM-11, mordenite [P. Chu, G. T. Kuhl, Ind. Eng. Chem. Res. 26 (1987) 365.; L. M. Tan and B. H. Davis, Appl. Catal., 53 (1989) 263.; and S. I. Pien and W. J. Hatcher, Chem. Eng. Comm., 93 (1990) 257.]}, montmorillonite [J. M. Adams, K. Martin, R. W. McCabe and S. Murray, Clays & Clay Miner., 34 (1986) 597. M. P. Atkins, J. Williams, J. A. Ballantine, J. H. Purnell, Eur. Pat. Appl. EP 284397 A1 (1988)], acidic TiO.sub.2 [J. K. Knifton and N. J. Grice, U.S. U.S. Pat. No. 4,822,921], acidic alumina [F. Ancilloti, M. M. Mauri, E. Pescarollo and I. Romagnoni, J. Mol. Catal. 4 (1978) 37.], Kieselguhr [U.S. Pat. No. 3,906,054], heteropoly acid salt (HPA salt) [J. S. Kim, G. Seo, N. C. Park and H. Niiyama, Appl. Catal. 37 (1988) 45.], HPA/TiO.sub.2 [J. F. Knifton, U.S. Pat. No. 4,827,048 A (1989)], FCSA (supported fluorocarbon sulfonic acid polymer) [L. M. Tan and B. H. Davis, Appl. Catal., 53 (1989) 263.; and J. D. Weaver, E. I. Tasset and W. E. Fry in "Catalysis 1987--Studies in Surface Science and Catalysis, Vol. 38" J. W. Ward ed., Elsevier, Amsterdam, p. 24, 1988.,14], PPA (a phenylphosphonic acid resin) [D. E. Pearson, U.S. Pat. No. 4,133,838 (1979).], Nafion-H [F. J. Waller and R. W. van Scoyoc, Chemtech, July (1987) 438.] and a sulfonic acid type resin Amberlyst-15 (A-15) [A. Ali and S. Bhtia, Chem. Eng. J., 44 (1990) 97.], etc. Among them, A-15 is currently used in the chemical industry for manufacturing MTBE.
Amberlyst-15 resin is a macroreticular cation-exchanger and a sulfonic acid type based on a styrene-divinylbenzene copolymer. The catalyst was reported to be unstable above 90%, and overheating caused release of sulfonic and sulfuric acids [N. W. Frish, Chem. Eng. Sci. 17 (1962) 735.]. A-15 even used in a lower temperature reaction will release a small amount of acidic material to the reaction products [S. Yamanaka and M. Koizunii, Clays & Clay Miner. 23 (1975) 477.], which may in turn cause the engine corrosive.
3. Development of zirconium phosphate derivatives:
The research related to layered metal (IV) phosphate compounds and the derivatives thereof was started from 1964. In that year, Clearfield and Stynes first synthesized zirconium phosphate crystal, identified the synthesized compound as Zr(HPO.sub.4).sub.2.H.sub.2 O and presented the matrix structure of the crystal [A. Clearfield and J. A. Stynes, J. Inorg. Nucl. Chem., 26 (1964) 117].
In 1970's Yamanaka [S. Yamanaka, Inorg. Chem. 15 (1976) 2811 .] and Alberti et al. [G. Alberti, V. Costantino and N. Tomassini, J. Inorg. Nucl. Chem. 40 (1978) 1113.] synthesized a layered zirconium phosphate having a bridge-bonded organo substituent with a general formula of Zr(PO.sub.3 R).sub.2, such as Zr(PO.sub.3 C.sub.6 H.sub.5).sub.2. This compound is significant in: (i) a relatively low synthesizing temperature unlike the previous solid compounds which can only be synthesized at a high temperature, (ii) a structure similar to that of zirconium phosphate, i.e. the organo substituent being bridged-bonded the same way as the phosphate bond. These properties allow us to design various layered zirconium phosphates with different substituents above and below the zirconium plane thereof, e.g. Zr(RPO.sub.3).sub.x (R'PO.sub.3).sub.2-x, wherein R and R' are an organo substituent, H. OH or OR; and 0&lt;x&lt;2 [G. Alberti and U. Costrantina, J. Mol. Catal. 27 (1984) 235.]. Occidental Research Corporation in the European Patent Application No. 10,857 [P. M. DiGiacomo, M. B. Dines and V. E. Parziale, European Patent Appl. 0 010 857 A2 (1979).] claims a layered or amorphous organometallic inorganic polymer of the formula: M(O.sub.3 ZO.sub.x R).sub.n, wherein M is at least one tetravalent metal; Z is a pentavalent metal; R is one or more organo groups; and n is 2. Compound Zr(O.sub.3 PC.sub.6 H.sub.4 SO.sub.3 H).sub.2 is also covered by this broad claim; however, it was not synthesized in the specification. As a matter of fact, it cannot be synthesized in accordance with the solution synthesis method disclosed in the specification due to its high solubility in water.
DiGiacomo and Dines in 1982 reported a method of synthesizing a zirconium phosphate sulfophenylethylenephosphonate, Zr(O.sub.3 PC.sub.2 H.sub.4 C.sub.6 H.sub.4 SO.sub.3 H).sub.2, by using a corrosive HF acid, wherein the product obtained was non-crystalline and unstable [P. M. DiGiacomo and M. B. Dines, Polyhedron 1 (1982) 61]. In 1987, Yang and Clearfield utilized a similar method to synthesize a zirconium sulfophenylphosphonate having a phosphite group and an interlayer spacing of 16.1.ANG. [C. Y. Yang and A. Clearfield, React. Polym. 5 (1987) 13], which includes the following steps: dissolving a small amount of ZrOCl.sub.2.8H.sub.2 O in an aqueous HF acid solution to form zirconium fluoro complex; adding phosphorous acid and phenylphosphonic acid to the mixture while vigorously stirring; keeping the mixture in 60.degree. C. water bath for 3 days; removing the precipitate from the mixture and sulfonating the precipitate with oleum. Although this method can produce a zirconium sulfophenylphosphonate with a high crystallinity, it is only suitable to small scale production. In addition, a corrosive HF acid is used.
In 1990, Clerici et al. disclosed a replacement method to obtain zirconium phosphate sulfophenylphosphonate deposited on silica, which comprises depositing .alpha.-zirconium phosphate on silica; immersing the deposited silica in a sealed bottle together with sulfophenylphosphonic acid at a temperature of 80.degree. C. for 20 days [M. G. Clerici, G. Alberti, M. Malentacchi, G. Bellussi, A. Prevedello and C. Corno, Eur. Pat. Appl. 0 386 845 A1 (1990)]. The replacement method has a problem in controlling the composition of the product.
One of the present inventors, Soofin CHENG, and her co-workers in an article, entitled "Layered Group(IV) Metal Phosphates as Catalysts for MTBE Synthesis" Journal of the Chinese Chemical Society, 1991, 38, 529-534, disclose layered phenylsulfonic acid derivatives of zirconium phosphate prepared by mixing an aqueous solution of ZrOCl.sub.2 with excess H.sub.3 PO.sub.4 solution (1.5M). After stirring at room temperature for 12 hours, the white gel was centrifuged and washed with deionized water until free of Cl-ions. A crystalline sample was obtained by refluxing the gel in a H.sub.3 PO.sub.4 solution (4.5M) for 48 hours, which was confirmed by its X-ray diffraction pattern and IR spectrum to be .alpha.-zirconium phosphate [formulated as Zr(HPO.sub.4).sub.2.H.sub.2 O]. The phenyl phosphonate derivative was prepared by refluxing zirconium phosphate gel (25g) or .alpha.-zirconium phosphate in phenyl phosphonic acid solution (300 mL, 3M) for 2 hours, followed by washing and drying. The resultant solids were sulfonated with fuming sulfuric acid according to the procedures described by Yang and Clearfield [C. Y. Yang and A. Clearfield, React. Polym. 5 (1987) 13]. The phenylsulfonic acid derivatives of zirconium phosphate prepared by this method possess high acidity (-5.6&gt;Ho&gt;-8.2) and give high activity toward MTBE formation, but this method as well as the sulfonation methods mentioned above all suffer drawbacks as follows: (i) a very low yield of the layered solid product, e.g. most being lower than 50% based on Zr content, and (ii) it is very difficult in controlling the amount of sulfonic acid groups incorporated in the layered structure and thus the reproductivity is very poor.
An object of the present invention is to provide an improved method of preparing a sulfophenylphosphonate derivative of zirconium phosphite (which is also termed zirconium phosphite sulfophenylphosphonate in the text of this specification), which does not has the drawbacks of the prior art methods as mentioned above.
Another object of the present invention is to provide a method of using a sulfophenylphosphonate derivative of zirconium phosphite as an acid catalyst, in which the sulfophenylphosphonate derivative of zirconium phosphite has a catalytic activity higher than that of Amberlyst-15 (A-15) resin in MTBE synthesis reaction.